1. Field of the Invention
The present invention relates to a surface acoustic wave filter in which a plurality of surface acoustic wave resonators are mounted on a single piezoelectric substrate and more particularly, the present invention relates to a surface acoustic wave filter having a lattice-type circuit construction.
2. Description of the Related Art
Various surface acoustic wave filters have been proposed to function as a bandpass filter. Among these surface acoustic wave filters, a surface acoustic wave filter that is constructed to deal with balanced input and output has a lattice-type circuit construction.
FIG. 16 shows the circuit diagram of a surface acoustic wave filter of a lattice-type circuit construction. In the surface acoustic wave filter shown in FIG. 16, between a signal terminal 101 and a signal terminal 102, one series arm is constructed and a series-arm resonator 103 defining a series arm is connected thereto. Furthermore, also between a signal terminal 104 and a signal terminal 105, another series arm is constructed and a series-arm resonator 106 defining a series arm is connected thereto.
Furthermore, between the signal terminal 101 and the signal terminal 105, a lattice arm is constructed and a lattice-arm resonator 107 defining a lattice arm is connected thereto. Furthermore, also between the signal terminal 104 and the signal terminal 102, a lattice arm is constructed and a lattice-arm resonator 108 is connected to define the lattice arm.
In the above-described surface acoustic wave filter, any of the series-arm resonators 103 and 106 and the lattice-arm resonators 107 and 108 may be composed of a surface acoustic wave resonator having a pair of terminals. That is, as shown in FIG. 17, the surface acoustic wave resonator having a pair of terminals contains an IDT electrode 109 disposed in the middle in the propagation direction of a surface acoustic wave and reflectors 110a and 110b disposed on both sides of the IDT electrode 109 in the surface wave propagation direction.
One example of filtering characteristics of the above surface acoustic wave filter is shown in FIG. 18.
In Japanese Unexamined Patent Application Publication No. 7-288442, by making the resonant frequency of series-arm resonators substantially agree with the antiresonant frequency of lattice-arm resonators in a surface acoustic wave filter of a lattice-type circuit as described above, it is understood that a filtering characteristic as shown in FIG. 19 can be obtained. That is, it is understood that the bandwidth can be expanded and that the steepness of attenuation characteristics can be increased in the vicinity of the passband.
On the other hand, Japanese Unexamined Patent Application Publication No. 9-289434 discloses a surface acoustic wave filter where a plurality of stages of surface acoustic wave filters having a lattice-type circuit construction are cascaded and where the capacitance ratio between a series arm and a parallel arm at each stage is made different. Here, it is understood that, because of the above-described construction, the steepness of filtering characteristics is increased in the vicinity of the passband and a high attenuation value can be obtained in the frequency region spaced away from the passband. For example, when a surface acoustic wave filter of a two-stage construction is considered, by making the capacitance ratio between the series-arm resonator and the lattice-arm resonator equal in a first stage, a filtering characteristic as shown in FIG. 18 can be obtained. Furthermore, in a second-stage filter, by making the capacitance ratio between the series-arm resonator and the lattice-arm resonator 1:2, a filtering characteristic as shown in FIG. 19 can be obtained. Therefore, by combining these filtering characteristics, it is understood that the attenuation value as a whole becomes high and the steepness in the vicinity of the passband can be increased.
In the surface acoustic wave filter described in Japanese Unexamined Patent Application Publication No. 7-288442, when attempts were made to obtain a high attenuation value at frequencies away from the passband, there was a problem that the steepness of filtering characteristics is reduced in the vicinity of the passband. As a lattice-type circuit constitutes a bridge circuit, when the capacitance of a series-arm resonator is represented by Z1 and the capacitance of a lattice-arm resonator is represented by Z2 and only when the balance condition of Z1=Z2 is met, the output signal becomes zero and an attenuation pole is caused. Therefore, when attempts are made to achieve an attenuation value at frequencies away from the passband high, the closer to 1:1 the ratio between the electrode capacitance Z1 of the lattice-arm resonator and the electrode capacitance Z2 of the series-arm resonator, the more desirable.
However, when the ratio of electrode capacitance between the lattice-arm resonator and the series-arm resonator is made 1:1, like the filtering characteristic shown in FIG. 18, there was a problem that no attenuation pole is produced in the vicinity of the passband and that the steepness is deteriorated around the passband.
On the other hand, as described in Japanese Unexamined Patent Application Publication No. 7-288442, when the ratio of electrode capacitance between the lattice-arm resonator and the series-arm resonator is changed from 1:1, like the filtering characteristic shown in FIG. 19, attenuation poles are produced around the passband and the steepness of filtering characteristics is improved. However, in this case, there was a problem that the attenuation value becomes low at frequencies away from the passband.
In the prior art described in Japanese Unexamined Patent Application Publication No. 9-289434, although the above problem is solved, because of a plurality of stages of surface acoustic wave filters of a lattice-type circuit construction are cascaded, the whole device including all of the surface acoustic wave filters becomes very large. Furthermore, because of the construction where a plurality of stages are connected, there was a problem that loss increases in the passband.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a lattice-type circuit construction that solves the above-mentioned problems with the prior art, the chip size is not increased to be a large size, loss in the passband is prevented from increasing, and the steepness in filtering characteristics is superior.
The surface acoustic wave filter according to a preferred embodiment of the present invention includes a piezoelectric substrate and a plurality of surface acoustic wave resonators mounted on the piezoelectric substrate. The plurality of surface acoustic wave resonators are connected so as to constitute a lattice-type circuit having series-arm resonators and lattice-arm resonators. Each capacitance ratio xcex3 value of the lattice-arm resonator and the series-arm resonator is different from each other.
In another preferred embodiment of the present invention, parallel capacitors are added to at least either of the lattice-arm resonators and the series-arm resonators so that the xcex3 values of the lattice-arm resonator and the series-arm resonator are made different from each other.
In another preferred embodiment of the present invention, IDT electrodes are provided and arranged such that in at least either of the lattice-arm resonators and the series-arm resonators, withdrawal weighting is performed so that the xcex3 values of the lattice-arm resonator and the series-arm resonator are different from each other.
In another preferred embodiment of the present invention, the difference in electrode capacitance between the series-arm resonator and the lattice-arm resonator including an external element connected to each resonator is preferably within about xc2x110% of the electrode capacitance of the lattice-arm resonator.
In a surface acoustic wave filter according to preferred embodiments of the present invention, because a lattice-type circuit construction is used and the xcex3 value of lattice arms is different from the xcex3 value of series arms, the steepness of filtering characteristics is greatly increased in the vicinity of the passband. For example, when the xcex3 value of lattice arms is higher than the xcex3 value of series arms, the steepness of filtering characteristics is significantly increased on the lower frequency side of the passband, and when the xcex3 value of series arms is higher than the xcex3 value of lattice arms, the steepness of filtering characteristics is significantly increased on the higher frequency side of the passband.
Furthermore, in preferred embodiments of the present invention, since the steepness of filtering characteristics can be increased by only controlling the xcex3 value of the lattice arms and series arms, it is not necessary to use an arrangement having a plurality of stages. Therefore, it is possible to avoid larger chip sizes and increased losses in the passband.
Consequently, according to preferred embodiments of the present invention, it is possible to provide a surface acoustic wave filter which has a very small size, a very small insertion loss, and excellent steepness of filtering characteristics.
Furthermore, it is possible to make the attenuation value high enough in the frequency region away from the passband.
In preferred embodiments of the present invention, when parallel capacitors are added to at least either lattice-arm resonators or series-arm resonators, it is possible to make the xcex3 value of the resonators having capacitors added thereto different from the xcex3 value of the resonators having no capacitors added thereto and to increase the steepness of filtering characteristics according to preferred embodiments of the present invention.
Furthermore, in preferred embodiments of the present invention, when the electrode fingers of at least either of the IDT electrodes of lattice-arm resonators and the series-arm resonators are thinned out, as the xcex3 value is made different from the other by the withdrawal weighting, the steepness of filtering characteristics can be increased according to preferred embodiments of the present invention.
Moreover, in preferred embodiments of the present invention, when the difference between the electrode capacitance of lattice-arm resonators and the electrode capacitance of series-arm resonators is within about xc2x110% and is more preferably within about xc2x15% of the electrode capacitance of the lattice-arm resonators, because the electrode capacitances of both resonators are close in value to each other, the attenuation value can be made high enough in the frequency region away from the passband.
Other features, characteristics, advantages and elements of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.
For the purpose of illustrating the present invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown.